Toxic effects on immune system development may seriously impair human health, but immunotoxicity is rarely considered in risk assessment of environmental chemicals. A main problem is the absence of systematic epidemiological evidence. We have successfully established birth cohorts for prospective studies in a unique fishing population exposed to marine contaminants - the Faroe Islands - and we have demonstrated that vaccine responses in children can be applied as a model of immunotoxicity associated with polychlorinated biphenyl (PCB) exposure. This competitive renewal application extends the follow-up of the same birth cohort of >600 children for sustained serum concentrations of antibodies against vaccines and in regard to allergic disease up to adolescence. In a second birth cohort of 500 children recently established, we will test whether the immunotoxicant exposure at age 18 months represents a main window of vulnerability in regard to the subsequent serum antibody concentrations. Further, we will explore the impact of past and cumulated exposure levels on responses to a vaccine booster, as reflected by specific antibody concentrations and by changes in lymphocyte populations and cytokine production. Likewise, clinical evidence of allergy development, including serum concentrations of total and specific IgEs against common antigens, will be evaluated in regard to exposures during prenatal and postnatal time windows. In addition to already confirmed immunotoxicants PCBs and methylmercury, the study will examine the possible immunotoxicity of the total AhR (dioxin-related) activity in serum and of perfluorinated compounds and butyltins. Vaccine antibody assays for tetanus and diphtheria toxoids will be expanded to include Pneumococcus polysaccharide antigens. Because this study relies on two existing birth cohorts with banked blood samples and background information already collected - in part financed by other sources - the proposed research will cover a time span of 14 years to allow consideration of exposures incurred prenatally and outcomes followed until adolescence. The cumulated evidence will therefore likely be of substantial importance for the understanding of developmental immunotoxicity in humans and for risk assessment of immunotoxic agents.